The invention relates to a method of and apparatus for forging a hollow blank member, and more particularly, to such method and apparatus which is preferred for use in the manufacture of a rack.
In the prior art, there are a number of techniques proposed which may be used to form a rack from a hollow rod in order to reduce the weight of the rack. A rack blank is formed from a hollow rod, and subsequently, a steel rod of a size which is commensurate with the internal diameter of the hollow section is fitted into a portion of the rack blank where rack teeth are to be shaped, as disclosed in Japanese Laid-Open Patent Applications No. 195,960/1982 and No. 206,546/1982. Alternatively, a material of indefinite configuration such as sand may fill the hollow interior of the rack blank, as disclosed in Japanese Laid-Open Patent Application No. 93,537/1983. The resulting rack blank is subsequently mounted in a recess of a lower die, and a punch is used to cause a plastic deformation of a region of the rack blank which is to be machined, thus forming rack teeth.
With these conventional techniques, if the steel rod is used to fill the interior of the rack blank, the steel rod must be left within the portion of the rack blank where the teeth are shaped after the completion of the rack teeth. Alternatively, if sand is employed, a stop member which is used to hold the quantity of sand within the hollow interior must be left within the shaped teeth portion of the rack blank. In either instance, the steel rod or the stop member which is left within the rack destroys the very purpose of reducing the weight of the rack.
A conventional apparatus for manufacturing a rack is illustrated in FIGS. 1(a) and (b). An arrangement shown in FIG. 1 (a) represents a forging apparatus of so-called closed type in which a rack blank 2 which is mounted in a recess within a lower die 1 is pressed by an upper die 3 which tightly fits in the recess, thereby forming rack teeth. This arrangement requires that the rack blank 2 be previously finished to a high accuracy since there is no place to relieve the metal of the rack blank 2 during its plastic deformation. An arrangement shown in FIG. 1 (b) illustrates a forging apparatus of so-called semi-closed type in which a rack blank 2' is mounted in a recess of a lower die 1', with the rack blank partly exposed out of the lower die 1'. An upper die 3' has a recess in its lower surface in which a tooth profile is defined, and is fitted over the exposed portion of the rack blank 2. When the die 3' is pressed against the latter, a clearance is left between the upper and lower dies 1', 3' to provide a space where the metal may be relieved during the plastic deformation, thus forming burrs 2'a. This arrangement eliminates the need for a high accuracy of previously machining the rack blank 2', but disadvantageously results in a degraded yield since a subsequent deburring operation is required (see FIG. 1(c)).